Vehicular rearview mirrors, especially for the exterior of an automobile or truck, are broadly classified as either spectrally nonselective, i.e., metallic or silvery in appearance, or spectrally selective, i.e., those which use light interference to enhance reflectance in some portion of the visible wavelength spectrum relative to other portions. For example, a first surface, chromium coated, glass mirror is a spectrally nonselective or metallic appearing mirror. Various types of commercially available, anti-dazzling, glare reducing blue mirrors are exemplary of spectrally selective mirrors.
Spectral selectivity makes the reflectance from the mirror dependent on the light source which projects light incident on the mirror as well as on the detector which senses the reflected light rays. In vehicular applications, the light source of most interest is usually a headlight while the detector is the human eye. The luminous reflectance of rearview mirrors is measured by using a light source which models light from a headlight and by using a detector with a filter which mimics the spectral selectivity of the human eye when in its day-adapted (photopic) mode. Such measurements of luminous reflectance are performed in accordance with SAE (Society of Automotive Engineers) Recommended Practice J964a for measurement of rearview mirror reflectivity. The human eye adapts to various levels of ambient light by changing its sensitivity to various colors. During the day, when light is abundant, human eye sensitivity is highest in the green spectral regions. As the light level drops, however, the peak eye sensitivity moves toward the shorter blue wavelengths. Since headlights have a spectral emission that is strong in longer yellow wavelengths but weak in blue, a glare reducing or anti-dazzling mirror which optimizes low light vision should accentuate reflectance in the blue regions where the eye is most sensitive but reduce reflectance in the yellow regions thereby reducing headlight reflectance. Such a mirror is, therefore, blue in color.
In addition, the optimal vehicular mirror which reduces glare and provides an anti-dazzling effect also has sufficient luminous reflectance to provide an image which is bright enough that the driver can quickly, accurately and easily gather information about the environment even in low light level conditions, but not so bright, i.e. not greater than 60% luminous reflectance or thereabouts, as to act as a source of glare from following headlights at night. Moreover, governmental regulations such as Federal Motor Safety Standard 111 in the United States require a mirror luminous reflectance of at least 35%. Likewise, in Europe, European Economic Community Council Directive 71/127/EEC requires a similar reflectance of at least 40% for mirrors. Thus, it is desirable that a rearview mirror have a luminous reflectance in the 35% to 60% range and that it be spectrally selective with the spectral selectivity being such that the mirror is more strongly reflecting in the blue/green region of the visible spectrum below about 560 nanometers than it is reflecting in the yellow/red region above 560 nanometers so that the mirror has a blue tint as viewed in reflectance.
Beyond headlight glare reduction in the yellow/red spectral regions and enhanced reflectivity of blue light where the human eye is sensitive at night, use of blue mirrors on vehicles has another advantage. Blue is perceived by consumers to be an aesthetically pleasing, relaxing color. Consequently, blue mirrors have found consumer acceptance and approval on vehicles. For all these reasons, spectrally selective blue automotive rearview mirrors of luminous reflectance in the 35% R to 60% R range have been widely used, particularly as exterior mirrors on luxury vehicles.
In order to meet the above objectives, numerous types of spectrally selective blue mirrors have been used in the past. Many prior mirrors, such as those disclosed in U.S. Pat. No. 4,673,248 and West German No. DE 34 360 16 have used relatively thick layers of opaque or near opaque metal having a luminous or bulk reflectance below about 80% such as alloys of nickel/chrome, aluminum/copper, nickel/cobalt, tin/copper or singular metals such as chromium, titanium, zinc or tin. Such metals are typically combined with various other layers, usually including dielectric materials. Metals having high bulk reflectance over 80% such as aluminum or silver in opaque layers have been avoided in spectrally selective rearview mirrors because such mirrors often have poor spectral selectivity. The use of the above-mentioned different alloys or singular metals such as chromium, titanium, zinc or tin which have lower total reflectances is necessary to avoid such loss of spectral selectivity. Hence, such mirrors often require more exotic metals or alloys, thicker metal layers and longer layer coating times resulting in more expensive products. Further, such mirrors have often failed to simultaneously achieve a commercially desirable blue spectral selectivity and maintain at least 35% luminous reflectance and particularly so in constructions where the thin film coatings are located behind, and are protected by, the substrate.
Accordingly, while the value of spectrally selective, blue mirrors has been previously recognized in the vehicular industry, a need has remained for a commercially acceptable, economically produced mirror which simultaneously achieves and combines luminous reflectance meeting minimum safety standards in the U.S.A. or Europe with good glare reduction and appreciable spectral selectivity especially in the blue visible wavelength regions of the spectrum to provide for enhanced low light visibility.